1. Field of the Invention
The present invention relates to an image forming apparatus in which an unfixed image is formed by transferring an image from an image bearing member to a transfer material supported by a transfer material support member and an image is obtained by fixing said unfixed image to the transfer material by fixing means.
2. Related Background Art
There is already known an image forming apparatus in which an unfixed toner image is formed by transferring a toner image, formed on a photosensitive member, to a transfer material supported by a transfer material support member such as a transfer drum or a transfer belt, and said unfixed toner image is fixed, by heat and pressure, onto the transfer material by a pair of fixing rollers.
The fixing rollers of such image forming apparatus are generally subjected to oil application on the surface, in order to extend the service life of the fixing rollers themselves and to prevent offsetting of the fixed toner to the rollers.
Consequently, the oil is inevitably deposited, though in a small amount, onto the transfer material which has passed the fixing position and on which the unfixed toner image has been fixed. The oil deposited on a part of the transfer material, not bearing the toner image thereon, is absorbed in the transfer material, but that deposited on the toner image cannot be absorbed. Consequently, in case of two-side (both-face) image formation by supporting a transfer material, bearing a fixed toner image on a first face, on the transfer material support member and forming an image on the second face of said transfer material, the oil deposited on said transfer material is transferred, through the transfer material support member, to the photosensitive member and hinders the image formation for the next transfer material.
More specifically, the toner is deposited in an area of the photosensitive member that should remain free of toner, by the adhesive force of the oil deposited onto the photosensitive member, thus forming fog. Also in an area which should appear as solid black, there may be formed a paler image by the insufficient toner transfer from the photosensitive member to the transfer material, again due to the adhesive force of the oil.